The invention relates to the field of wavelength multiplexed optical communications systems, such as wavelength division multiplexed (WDM) systems, and more particularly to the management and control of the chromatic dispersion of wavelength multiplexed optical signals.
Optical communications systems are a substantial and fast-growing constituent of communications networks. The expression xe2x80x9coptical communications systemxe2x80x9d, as used herein, relates to any system which uses optical signals to convey information across an optical medium. Such optical systems include, but are not limited to, telecommunications systems, cable television systems and local area networks (LANs). Optical systems are described in Gower, Ed. Optical Communications Systems, (Prentice Hall, N.Y.). Currently, the majority of optical communication systems are configured to carry a single optical channel having a narrow wavelength spectral band over one or more optical waveguides. To convey information from a plurality of sources, wavelength division multiplexing (WDM) is now used. In a WDM system a plurality of optical signals, each typically having a narrow wavelength spectral band, each band being centered on a different wavelength, are carried over a single optical guide.
A typical optical network comprises a plurality of nodes linked by a number of different optical paths for the carriage of WDM signals therebetween. Typically, each path will introduce chromatic dispersion into the signal components passing through it. The term chromatic dispersion is used here to indicate the undesirable effect where components of an optical signal following an optical path experience a different delay according to their wavelength. The differential delay xcex94t introduced between two signal components at a wavelength spacing of xcex94xcex=xcex1xe2x88x92xcex2 may be expressed mathematically as xcex94t=t1xe2x88x92t2 where t1 is the delay experienced by a first signal component at wavelength xcex1 and t2 is the delay experienced by a second signal component at wavelength xcex2. Chromatic dispersion is commonly introduced by conventional optical fibre. The chromatic dispersion D of an optical path, e.g. optical fibre, (i.e. the tendency of that path to introduce differential delay) may be expressed mathematically as D=xcex94t/L.xcex94xcex, where xcex94t is the differential delay introduced between a pair of optical signal components at a wavelength separation of xcex94xcex over a length L of the path. A typical value for D for optical fibre in use today is 16 pS/(nm.km).
One problem experienced in optical communication systems is the wide variation in the propagation time or delay experienced by component parts of optical signals following routes through the network according to their wavelength. This leads to a corresponding elongation or spreading of a transmitted data pulse as it passes through the network. In order to overcome this problem there is a need for a means to selectively introduce at a point in the network complementary delays into components of optical signals according to their wavelength.
A prior art method for applying a different delay to various components of a WDM optical signal (which comprises a plurality of signals each having a different optical spectrum, each occupying a different optical channel with a different wavelength band) in an optical network has been to reflect the signal using a long fibre Bragg grating. This method has the disadvantage that it is only capable of introducing a fixed delay. In addition this method requires the creation of a very long grating with a very low chirp (i.e. the resonant frequency of the elements of the grating varies very slowly along the length of the grating) in order to ensure that regions of the grating have elements with a resonance frequency which matches each channel of the WDM signal, and that the variation in resonance frequency along the length of the grating is arranged to reflect the separate elements of the spectrum of the signal so as to compensate for the accumulated differential group delay. These long, gradually chirped, fixed-length gratings have a fixed gradient of delay against spectral width. They are therefore well suited to compensating for the chromatic dispersion for a particular fixed path length. If the different channels of an optical network are routed over different paths through the network then different gradients of delay/spectral width would be required for each of the channels. This requirement for a long grating with a small, linearly varying change in resonant frequency with axial position in the fibre introduces the danger of a local deviation in the chirp where the change in the local resonance frequency reverses for a section of the grating. This would have the disadvantage of introducing an unwanted Fabry-Perot etalon into the grating, which would result in distortion of the signal and impairment in the detection process.
The present invention provides a system for compensating for chromatic dispersion of an optical signal in which the system comprises a delay means for introducing a differential delay between two spectrally separate components of the optical signal by selectively delaying components of the optical signal by reflection according to the wavelengths of the components; in which the system comprises adjusting means for adjusting the differential delay introduced between two of the spectrally separate reflected components.
In a preferred embodiment the present invention provides a system for also compensating for chromatic dispersion of a plurality of spectrally separate optical signals in a single optical path; in which the system comprises a plurality of the delay means, one per signal; and in which the system comprises adjusting means for adjusting the differential delay introduced between two of the reflected components of each of the optical signals.
In a preferred embodiment the present invention provides a system for selectively delaying one of the plurality of optical signals relative to a second one of the plurality.
In a preferred embodiment the present invention provides a system for selectively providing a group delay of one of the plurality of optical signals at one delay means and to provide a further group delay of the same or opposite sign at a second delay means to provide a greater range of group delay which includes zero overall delay.
References to positive and negative delay cover the alternative cases, i.e. whether the introduced delay increases or decreases with increasing wavelength.
In a preferred embodiment the present invention provides a system comprising an interferometer for separating the plurality of spectrally separate optical signals into a first and a second optical guide; in which the first and second optical guides each comprise the delay means.
In a preferred embodiment the present invention provides a system comprising signal bit error rate measurement means.